Fix for #15 and OpenCV 4.0 compatibility

[harveybia]

• Fixed #15
• Fix compile errors for deprecated flags and includes in OpenCV 4.0
• Fixed gitignore for ignoring log files in FAST-LIO
• Moved loam-livox rviz config default viewing port to center

Tested with Livox Avia on ubuntu 20.04 noetic with OpenCV 4.0

[gisbi-kim]

@harveybia thanks!! merged.